1. Field of the Invention
The present invention relates to an improvement in input and output discs for a toroidal type continuously variable transmission used as a transmitting unit of a transmission of an automobile or transmissions for various industrial machines, and a method for manufacturing or working such discs.
2. Related Background Art
Use of various toroidal type continuously variable transmissions as shown in FIGS. 5 and 6 as transmission for automobiles has been investigated. In such a toroidal type continuously variable transmission, for example, as disclosed in Japanese Patent Application Laid-Open No. 11-141637 (1999), an input disc 2 is supported coaxially with an input shaft 1, and an output disc 4 is secured to an end of an output shaft 3 disposed coaxially with the input shaft 1. Within a casing containing the toroidal type continuously variable transmission, there are provided trunnions 6 rocked around pivot shafts 5 arranged at twisted positions with respect to the input shaft 1 and the output shaft 3.
The trunnions 6 are provided at their outer surfaces with the pivot shafts 5 at both ends thereof. Further, the trunnions 6 support proximal ends of displacement shafts 7 at their centers so that inclination angles of the displacement shafts 7 can be adjusted by rocking the trunnions 6 around the pivot shafts 5. Around the displacement shafts supported by the trunnions 6, power rollers 8 are rotatably supported. The power rollers 8 are pinched between the input disc 2 and the output disc 4. Opposed inner surfaces 2a, 4a of the input and output discs 2, 4 have concave surfaces obtained by rotating an arc or a similar curved line around the input and output shafts 1, 3 respectively. Peripheral surfaces 8a of the power rollers 8 formed as spherical convex surfaces abut against the inner surfaces 2a, 4a. 
Between the input shaft 1 and the input disc 2, there is provided an urging device 9 such as a loading cam which serves to elastically bias the input disc 2 toward the output disc 4. The urging device 9 comprises a cam plate 10 rotated together with the input shaft 1, and a plurality (for example, four) of rollers 12 held by a holder 11. One surface (left surface in FIGS. 5 and 6) of the cam plate 10 has a cam surface 13 as a circumferential uneven surface, and an outer surface (right surface in FIGS. 5 and 6) of the input disc 2 has a similar cam surface 14. The plurality of rollers 12 are supported for rotation around axes radially to the center of the input shaft 1.
In use of the toroidal type continuously variable transmission having the above-mentioned construction, when the cam plate 10 is rotated as the input shaft 1 is rotated, the plurality of rollers 12 are urged against the cam surface 14 provided on the outer surface of the input disc 2 by means of the cam surface 13. As a result, the input disc 2 is urged against the plurality of power rollers 8, and, at the same time, the input disc 2 is rotated due to the engagement between the cam surfaces 13, 14 and the plurality of rollers 12. The rotation of the input disc 2 is transmitted to the output disc 4 via the plurality of power rollers 8, thereby rotating the output shaft 3 secured to the output disc 4.
In a case where a rotational speed ratio (transmission ratio) between the input shaft 1 and the output shaft 3 is changed, first of all, when speed reduction is achieved between the input shaft 1 and the output shaft 3, by rocking the trunnions 6 around the pivot shafts 5, as shown in FIG. 5, the displacement shafts 7 are inclined so that the peripheral surfaces 8a of the power rollers 8 abut against a portion near the center on the inner surface 2a of the input disc 2 and a portion near the periphery on the inner surface 4a of the output disc 4, respectively. On the other hand, when speed increase is achieved, by rocking the trunnions 6 around the pivot shafts 5, as shown in FIG. 6, the displacement shafts 7 are inclined so that the peripheral surfaces 8a of the power rollers 8 abut against a portion near the periphery on the inner surface 2a of the input disc 2 and a portion near the center on the inner surface 4a of the output disc 4, respectively. If the inclination angles of the displacement shafts 7 are selected to have middle values between FIG. 5 and FIG. 6, a middle transmission ratio can be obtained between the input shaft 1 and the output shaft 3.
Fundamental structure and function of the toroidal type continuously variable transmission are as mentioned above, and a more concrete construction of such a toroidal type continuously variable transmission is well-known, as disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 11-141637. Further, a toroidal type continuously variable transmission of so-called double cavity type in which two input discs and two output discs are disposed side by side in a power transmitting direction to increase a power to be transmitted is also well-known, for example, as disclosed in Japanese Patent Applications Laid-Open Nos. 1-229158 (1989) and 4-69439 (1992).
Since the more concrete construction of such a toroidal type continuously variable transmission is well-known and does not directly relate to the gist of the present invention, explanation thereof will be omitted. However, in any case , configurations and features (surface roughness) of the inner surfaces 2a, 4a of the input and output discs 2, 4 must be formed correctly as desired. The reason is that the inner surfaces 2a, 4a must transmit great torque while abutting against the peripheral surfaces 8a of the power rollers 8 and the contacting conditions between the surfaces 2a, 4a and 8a are rolling contact including slip due to spin.
Accordingly, in order to extend a service life of the surfaces 2a, 4a, 8a against rolling fatigue and to allow great torque to be transmitted, the contact portions of the surfaces 2a, 4a, 8a must have traction oil films each having a proper thickness. The accuracy of configurations of the inner surfaces 2a, 4a are required for optimizing the contacting conditions between the surfaces 2a, 4a and 8a, and the regulation of surface roughness of the surfaces 2a, 4a, 8a to the desired value is required for optimizing the thickness of the traction oil films.
To this end, conventionally, there has been proposed a finishing operation in which the inner surfaces 2a, 4a of the input and output discs 2, 4 are polished by a grindstone having a desired feature (surface roughness) to finish the configurations and features (surface roughness) of the inner surface 2a, 4a as desired. The grindstone which can be used in such a finishing operation is disclosed, for example, in Japanese Patent Application Laid-Open No. 8-252770 (1996). FIGS. 7 and 8 show a grindstone 15 and a method for effecting a finishing operation on an inner surface 16a of a disc 16 used as an input disc or an output disc of a toroidal type continuously variable transmission by using this grindstone 15, disclosed in the above patent gazette.
A working surface 17 of the grindstone 15 is an arc surface having a sectional configuration corresponding to a sectional configuration of the inner surface 16a. When the inner surface 16a is polished by such a grindstone 15, the disc 16 is rotated around a center axis shown by the dot and chain line a in FIG. 8 while urging the working surface 17 against the inner surface 16a. In this case, a shoe is slidingly contacted with a portion of a cylindrical surface 18 formed in outer periphery of the disc 16 at a position opposed to the grindstone 15 in a radial direction, thereby preventing radial displacement of the disc 16 (out-of-center during the working) in opposition to the urging force of the grindstone 15.
In case of the above-mentioned conventional method for working the disc for the toroidal type continuously variable transmission, the cylindrical surface 18 formed in the outer periphery of the disc 16 must be worked with high accuracy. The reason is that, if a center axis of the cylindrical surface 18 does not coincide with a center axis of the inner surface 16a, the configuration of the inner surface 16a cannot be formed as desired. On the other hand, in an assembled condition of the toroidal type continuously variable transmission, since the cylindrical surface 18 does not contact with any other member, in a viewpoint of maintenance of performance of the toroidal type continuously variable transmission itself, it is not required that the working accuracy of the cylindrical surface 18 be higher. Accordingly, in order to reduce the manufacturing cost of the toroidal type continuously variable transmission, it is desirable to provide a method for effecting the working operation for the inner surface 16a with high accuracy without maintaining high accuracy of the cylindrical surface 18.
Further, since the shoe is slidingly contacted with the cylindrical surface 18, if a working speed (rotational speed of the disc 16) is increased, seizure will occur in the slidingly contacting portion and the sliding trace of the shoe will be generated. For this reason, the working speed cannot be increased, with the result that the working time is lengthened, thereby increasing the manufacturing cost.
Further, depending upon the structure of the disc having the inner surface to be subjected to the finishing operation, it may be difficult to perform the above-mentioned method for working the disc for the toroidal type continuously variable transmission. For example, the above-mentioned Japanese Patent Laid-Open No. 11-141637 discloses, as shown in FIG. 9, a technique in which an output gear 19 is directly formed on an outer periphery of an output disc 4A of a toroidal type continuously variable transmission of double cavity type. In such a design, the shoe cannot be engaged with the outer periphery of the output disc 4A. Thus, in order to work or form such an output disc 4A, the output gear 19 must be formed on the outer periphery of the output disc 4A after both axial surfaces of the output disc 4A were finished. Such a working operation is not desirable because of not only increased cost but also possibility of damaging the already finished both surfaces of the output disc during the formation of the output gear 19.